Frequency modulated wave detector



AM DETEC'TU v April 21, 1942. s. HUNT FREQUENCY MODULATED WAVE DETECTORFiled July 2, 1940 DISCRIMINATO T A N m 0 U AM w mag 0 0T 4/ N 7 W 4 7W+M0 M H w 2 I. C J Y a j M 1 B. 1Y

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smurf SOURCE ATTORNEY Patented Apr. 21, 1942 FREQUENCY MODULATED WAVEDETECTOR Seymour Hunt, Jackson fleights, N. Y., assignor to RadioCorporation of America, a corporation of Delaware Application July 2,1940, Serial No. 343,537

3 Claims.

My present invention relates to frequency modulated carrier wavedetectors, and more particularly to novel and improved types of networksadapted to derive modulation voltage from frequency, or phase, modulatedcarrier waves.

One of the main objects of my present invention is to provide afrequency modulated (FM) wave detector wherein there is varied thevoltages of at least two control electrodes of an electron dischargetube at common frequencies corresponding to the mean, or center,frequency of an applied FM wave, and there being maintainedsubstantially a phase quadrature relation between the voltages of thecontrol electrodes whereby the output current of the detector tube isamplitude modulated (AM).

Another important object of my present invention is to derive themodulation from an FM wave by applying the latter to the controlelectrode of an electron discharge. tube, simultaneously varying thevoltage of a second control electrode of the tube at the mean frequencyof the FM wave, and producing modulation voltage in the tube outputcircuit by virtue of the space charge coupling between said controlelectrodes which are arranged in the same electron stream of said tube.

Another object of the invention is to provide an FM detector tube whichhas an oscillator section producing voltage whose frequency is the meanfrequency of received FM waves, a second control electrode beingarranged in the space current path to the tube output electrode andbeing energized by the FM waves in such phase that the modulationvoltage is developed in the output electrode circuit.

Still other objects of my invention are to improve generally thesimplicity and reliability of FM detectors, and more especially toprovide FM detectors capable of efficient operation and economicalconstruction.

The novel features which I believe to be char-' acteristic of myinvention are set forth in particularity in the appended claims; theinvention itself, however, as to both its organization and Fig. 2graphically shows the plate current variation of the discriminator tube,Fig. 3 illustrates a modification, Fig. 4 shows still anothermodification. Referring now to the accompanying drawing, wherein likereference characters in the different figures designate similar circuitelements, let it be assumed that the tube I in Fig. 1 is of the BA?type, the latter being a pentagrid tube usually employed for signalfrequency conversion to an intermediate frequency (I. F.). That type oftube generally comprises a cathode 2, a plate 3 and five successive gridelectrodes 4 to 8. The grid 5 usually consists of a pair of spaced rods,as is well known by those skilled in the art. In such a tube the platecurrent dependson the combined effects of the voltages of grids 4 and'1. Now, I have found that the plate current (Ip) can be varied inamplitude, in the manner shown in Fig. 2, if there is applied to grid 1a voltage of variable frequency, while grid 4 is maintained fixed at avoltage whose frequency is the center, or mean, frequency (in) of thefrequency deviation at grid I.

In other words, assume that there is applied to grid 1 a signal ofvariable frequency, such as an FM wave. Grid 4 may be varied at thecenter frequency of the wave by reactively coupling electrode 5 to theresonant tank circuit 9. The latter is tuned to h, and the coil thereofmay be coupled, as at M, ,to the coil in the positive voltage lead tothe anode, or oscillator plate, electrode 5. The grid leak condenser l0and leak resistor H are associated with oscillator grid 4 as shown so asto establish grid 4 at a proper negative bias for oscillationproduction. Hence, it will be seen that grid 4 has applied to it anunmodulated oscillation voltage whose frequency is fixed at is. Grids 6and 8 are positively biased,

and act to shield signal grid 1 from the oscillation grid 4.

The plate 3 includes in its positive voltage lead stantaneous frequencyvaries above and below )e in accordance with modulation signals appliedto a carrier frequency '(fc) at the transmitter. As the signal frequencyat grid 1 approaches the value fc, that is the fixed frequency ofoscillator grid 4, the plate current flow through circuit I? will changefrom its steady value.

When the; signal frequency is equal to Jc, the plate currentcharacteristic is adapted for detection of FM waves, since themodulation existing as a frequency variation of a constant amplitudecarrier has been converted into modulation existing as an amplitudevariation of a constant frequency carrier. Hence, it is merely necessaryto apply the voltage across circuit l2 to a rectifier of any well knowntype.

Thus, the diode M has its resonant input circuit I5 coupled tocircuitl2, circuit [5 being tuned to fc.

'I-heload resistor is, shunted by carrier bypass condenser l1, isarranged in the low potential side of the diode input circuit.Themodulation voltage developed across resistor l5'is utilizedin anaudio network, assuming the modulation on the-carrier to be ofaudiofrequency. The

direct current voltage across resistor [6 may be used for automaticvolume'control (AVC) of FM transmission tubes prior to tube l so as tocompensatefor fading ofthe carrier at the signal collector.

While the FM source feeding network l3 may be-of any known type,- it isgenerally the practice at'the present time to use a superheterodyne typeof receiver. Since the FM band covers a range of 42' to 50-megacycles(mc.), the collected- FM- signals are applied toa first-detector withlocal oscillations which differ from the center frequency of the FMwaves by adesired I. F. value generally chosen to 2.1 me. The I. F.energy is amplified by one or more I. F. am-' plifiers to a relativelyhigh level, and thenthe highly amplified I. F. energy is applied to alimiter stage which acts to eliminate any ampli tude variationproduoedin-theFM waves by noise or other causes. 'I'he limiter output,still at I. F., is applied to network l3- in'Fig. 1. Therefore, thevalue of fc will bethe I. F. value of 25.1 mo. The-A-VC voltage will, ofcourse; be-applied'to the I. F. amplifiers and prior tubes, if desired.

It is possible to provide the elements of rectifier M withinthe'envelope of tube I. As shownin Fig. 3- the cathode 2 has arrangedadjacent to it an auxiliary anode 2D; The latter has an electron streamflowing thereto which is independent of the stream to plate 3. Loadresistor i6 is provided between anode 'and cathode 2. The carriercoupling condenser 2| connects the high potential side of resonantcircuit 12 to the anodeZll. The FM source is schematically designated at22, while the local oscillation source-is schematically represented at23 as being connected to grid 4; the oscillator anode 5 having beenomitted.-

While the arrangements in Figs. 1 and Sfunction as discriminators, thecircuit in Fig. 4"is arranged to provide the modulation voltage directlyacross the load impedance of the plate circuit. In other words, in thearrangements of Figs. 1 and 3 the tube I and associated circuitsfunction to convert the FM waveinto an AM" wave, Whereas in the circuitof Fig. 4 there occurs simultaneous demodulation of the AM wave. Toaccomplish this effect there is'connecteda' resistor 30 in-the-positivevoltageleadto plate 3,

and the resistor is shunted by the I. F. carrier by-pass condenser 3|.The resistor-condenser network 494I in the grid circuit of tube I mayprovide amplitude variation limiting, if such limiting is necessary. Asthe carrier amplitude increases in level grid 1 becomes increasinglynegative due to resistor and will flatten off the output in the platecircuit.

While the following theoretical explanation of the physical phenomenaunderlying the operation of the invention is not believed essential, itis supplied to assist those skilled in the art better to comprehend theinvention. If e is the voltage developed across the tuned signal inputcircuit by. theoscillator voltage E, then the voltage e tends tomakegrid I (considering Fig. 1) either more or less positive, or negative,with respect to the cathode 2. This changes the 9111 of tube I. If theoscillator frequency is on the high side (above the value fc) of thesignal circuit frequency, then e is out of phase with E. Conversely,when the oscillator frequency is less than ,fc the voltage e is in phasewith E. Hence, there occurs a variation in gm or gain, of'tube I whichdepends on the relation between the voltage frequency at grid 1 and thatat grid 4. It should be pointed out that the voltage e is developed bythe virtual cathode" (or space charge coupling) which exists betweengrids 1 and 4. This cloud of electrons, often termed a virtual cathode,is caused by the negative bias of grid 1, and hence a negative voltagesource may be provided to establish grid T at such a negative potential.The virtual cathode varies at the oscillator frequency, and develops thevoltage e across the signal input circuit. When the. freload a timeconstant such that solely audio volt- Proper design of'theage willappear across 30. Q of the coils of the circuit will produce a-desirablefrequency spacing between the peaks 'of V the curve in Fig. 2.Furthermore, sincethe oscillator section of the tube. operates at afixed:

frequency, it is readily crystal-controlled. Inthe arrangement of Fig. 3it is possible to supply grids 1 and 4= with FM waves from a common.

While I have shown tube l as of the 6A7 type,.

it is to be clearly understood that there may be used a 6A8 type of tubein place thereof. In generaLthere may be used a tube which has av pairof spaced control grids in a common electron stream, the essentialrequirement being that the;

voltage applied to one grid is variable in frequency relative to theother, and that both voltages have a mean frequency and be so related asto cause the output. current to vary in amplitude in accordance withthe. instantaneous. frequency of the variable frequency voltage. In eachof Figs. 1, 3 and 4 the signal and oscillator grids may be interchangedwithout effect on the plate q 7 current characteristic.

It will be noted that the voltages applied to grids 4 and I must be inphase quadrature relation. The space charge coupling is a convenientmanner of producing a quadrature phase difference between the voltagesof grids 4 and I. It can be analytically shown that for a resistive loadin the circuit of grid 1 (and a tuned circuit at resonance may beregarded as a resistance) the induced voltage on grid 1 is 90 degreesout of phase with the voltage applied to grid 4. At the center frequencyof the FM waves the quadrature relation exists between the voltages ofgrids 4 and 1.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdepart ing from the scope of my invention, as set forth in the appendedclaims.

What I claim is:

1. In combination with an electron discharge tube having at least acathode, an output electrode and a pair of control electrodes arrangedin the electron stream between the cathode and output electrode, meansapplying a variable frequency signal of constant amplitude to onecontrol electrode, means applying a substantially constant frequencysignal of the center frequency value of said variable frequency signaland of substantially constant amplitude to the second control electrode,said applying means comprising an anode electrode located in said tube,a resonant circuit tuned to said constant frequency reactively couplingsaid anode and second cold electrode, and rectifier means connected tothe output electrode for deriving from the electron stream modulationvoltage proportional to the instantaneous frequency of said variablefrequency signal.

2. In combination with an electron discharge tube having a cathode, aplate and at least three cold electrodes arranged in the electron streamvide an oscillator section adapted to produce fixed frequencyoscillations of the mean frequency of said source, and an output circuitcoupled to said plate to develop a voltage whose amplitude varies inaccordance with the frequency variations of said source waves.

3. In combination with an electron discharge tube having a cathode, aplate and at least three cold electrodes arranged in the electron streambetween the cathode and plate, a source of substantially constantamplitude-frequency variable waves coupled to one of said coldelectrodes, a resonant network regeneratively coupling the cathode andthe other two cold electrodes to provide an oscillator section adaptedto produce fixed frequency oscillations of the mean frequency of saidsource, and an output circuit coupled to said plate to develop a voltagewhose amplitude varies in accordance with the frequency variations ofsaid source waves, said output cirpledbetween said wave source and saidone cold electrode. SEYMOUR HUNT.

